Let me just say that the penis does NOT have sexual orientation.
So while a straight guy wouldn't get hard from looking at dudes, if a guys sexual organ is getting simulated, it's going to get hard, whether you want it too or not. Just look at rape victims who feel horrible about themselves because their dicks got hard even though they were being violated.
That's about all I have to say. Oh, that and, a guy shows he's stimulated by erections, girls show it by wet *******. It seems that you guys think that erections are the only way to judge if someone's into it, and you can't judge whether or not a girl is into it. That's not true.